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PIC16C717/770/771

DS41120B-page 120

2002 Microchip Technology Inc.

12.2.4

ER MODE

For timing insensitive applications, the ER (External

Resistor) Clock mode offers additional cost savings.

Only one external component, a resistor connected to

the OSC1 pin and VSS, is needed to set the operating

frequency of the internal oscillator. The resistor draws

a DC bias current which controls the oscillation fre-

quency. In addition to the resistance value, the oscilla-

tor frequency will vary from unit to unit, and as a

function of supply voltage and temperature. Since the

controlling parameter is a DC current and not a capac-

itance, the particular package type and lead frame will

not have a significant effect on the resultant frequency.

Figure 12-3 shows how the controlling resistor is con-

nected to the PIC16C717/770/771. For REXT values

below 38 k

, the oscillator operation may become

unstable, or stop completely. For very high REXT values

(e.g. 1M), the oscillator becomes sensitive to noise,

humidity and leakage. Thus, we recommend keeping

REXT between 38 k

and 1 M.

FIGURE 12-3:

EXTERNAL RESISTOR

The Electrical Specification section shows the relation-

ship between the REXT resistance value and the oper-

ating frequency as well as frequency variations due to

operating temperature for given REXT and VDD values.

The ER Oscillator mode has two options that control

the OSC2 pin. The first allows it to be used as a general

purpose I/O port. The other configures the pin as CLK-

OUT. The ER oscillator does not run during RESET.

12.2.5

INTRC MODE

The internal RC oscillator provides a fixed 4 MHz (nom-

inal) system clock at VDD = 5V and 25

°C, see “Electri-

cal Specifications” section for information on variation

over voltage and temperature. The INTRC oscillator

does not run during RESET.

12.2.6

CLKOUT

In the INTRC and ER modes, the PIC16C717/770/771

can be configured to provide a clock out signal by pro-

gramming the configuration word. The oscillator fre-

quency, divided by 4, can be used for test purposes or

to synchronize other logic.

In the INTRC and ER modes, if the CLKOUT output is

enabled, CLKOUT is held low during RESET.

12.2.7

DUAL SPEED OPERATION FOR ER

AND INTRC MODES

A software programmable dual speed oscillator is avail-

able in either ER or INTRC Oscillator modes. This fea-

ture allows the applications to dynamically toggle the

oscillator speed between normal and slow frequencies.

The nominal slow frequency is 37 kHz. In ER mode, the

slow speed operation is fixed and does not vary with

resistor size. Applications that require low current

power savings, but cannot tolerate putting the part into

SLEEP, may use this mode.

The OSCF bit in the PCON register is used to control

Dual

Speed

mode.

See

the

PCON

When changing the INTRC or ER internal oscillator

speed, there is a period of time when the processor is

inactive. When the speed changes from fast to slow,

the processor inactive period is in the range of 100

to 300

S. For speed change from slow to fast, the pro-

cessor is in active for 1.25

S to 3.25 S.

RA7/OSC1/CLKIN

RA6/OSC2/CLKOUT

PIC16C717/770/771

REXT

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